Frequency modulator, frequency modulating method, and wireless circuit

ABSTRACT

A voltage controlled oscillator  1 , a variable frequency divider  2 , a phase comparator  3 , and a loop filter  4  form a Phase Locked Loop (PLL). A sigma-delta modulator  5  sigma-delta modulates data obtained by adding a fractional part M 2  of the frequency division factor data with modulation data X by using an output signal of the variable frequency divider  2  as a clock. An output signal of the sigma-delta modulator  5  is added to an integral part M 1  of the frequency division factor data, and the resultant data becomes effective frequency division factor data  13  of the variable frequency divider  2 . An output signal of the sigma-delta modulator  5  also becomes control data  14  after passing through a D/A converter  6 , a low-pass filter  7 , and an amplitude adjustment circuit  8 . The control data  14  is inputted into a frequency modulation terminal of the voltage controlled oscillator  1 . Therefore, it is possible to provide a frequency modulator that can use a reference signal source having no frequency modulation function, and perform modulation over a wide range of frequencies based on a digital modulation signal.

This application is a divisional application of application Ser. No.10/384,556, filed Mar. 11, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to frequency modulators, moreparticularly, a frequency modulator and a frequency modulating methodfor performing frequency modulation using a PLL (Phase Locked Loop), anda wireless circuit using the frequency modulator and the frequencymodulating method.

2. Description of the Background Art

As exemplary methods for performing frequency modulation using a PLL,the following two methods are known. In a first method, a referencesignal source is subjected to frequency modulation. In a second method,a modulation signal is supplied to a frequency control terminal of avoltage controlled oscillator. In general, the modulation signal used indigital communications, etc., has flat frequency characteristics fromlow to high frequencies within a predetermined band. Thus, in the casewhere frequency modulation is performed based on such a modulationsignal, flat characteristics have to be obtained when the modulationsignal spans over a range of low to high frequencies.

The above-described first method, however, requires a reference signalsource having a frequency modulation function, and makes it difficult tomodulate over a frequency band wider than a PLL loop bandwidth. In thiscase, a widened PLL loop bandwidth enables modulation over a wide rangeof frequencies. However, the widened loop bandwidth generallydeteriorates a C/N (Carrier to Noise Ratio) obtained at a point awayfrom zero by a width corresponding to the loop bandwidth. Thus, a PLLdoes not allow its loop bandwidth to be sufficiently widened.

On the other hand, the above-described second method makes it difficultto modulate over a frequency lower than the PLL loop bandwidth. In thiscase, a narrowed PLL loop bandwidth enables modulation over a lowerfrequency band. However, the narrowed loop bandwidth generally slows theloop response speed of the PLL. Thus, a PLL does not also allow its loopbandwidth to be sufficiently narrowed.

In order to solve the above-described problems, a third method that is acombined adoption of the above-described first and second methods isknown. FIG. 13 is a block diagram illustrating the structure of aconventional frequency modulator using the third method. The frequencymodulator shown in FIG. 13 includes a voltage controlled oscillator 1, avariable frequency divider 2, a phase comparator 3, a loop filter 4, anda reference signal source 10. The reference signal source 10 generates areference signal with a predetermined frequency. The voltage controlledoscillator 1, the variable frequency divider 2, the phase comparator 3,and the loop filter 4 form the PLL, which will be described below.

The variable frequency divider 2 divides the frequency of an outputsignal of the voltage controlled oscillator 1 based on providedfrequency division factor data M. The phase comparator 3 compares thephases of an output signal of the reference signal source 10 and theoutput signal of the variable frequency divider 2. The output signal ofthe phase comparator 3 is inputted into the voltage controlledoscillator 1 after passing through the loop filter 4. The voltagecontrolled oscillator 1 oscillates at a frequency appropriate to theoutput signal of the loop filter 4. This PLL performs feedback controlso as to keep a center frequency of the output signal of the voltagecontrolled oscillator 1 at a predetermined value. The reference signalsource 10 and the voltage controlled oscillator 1 are each provided witha frequency modulation terminal, to which an analog modulation signal Xais supplied.

FIG. 14 is an illustration showing frequency modulation characteristicsof the frequency modulator shown in FIG. 13. In FIG. 14, low-passcharacteristics shown in solid line represent the degree of modulationof the output signal against a modulation signal supplied to thereference signal source 10, and high-pass characteristics shown indashed line represent the degree of modulation of the output signalagainst a modulation signal supplied to the voltage controlledoscillator 1. In this case, it is possible to obtain frequencymodulation characteristics that are flat over a wide range offrequencies by adding the low-pass characteristics (solid line) and thehigh-pass characteristics (dashed line). Thus, even if a bandwidth(shaded portion) of a modulation signal is wider than a PLL loopbandwidth, it is possible to obtain good frequency modulationcharacteristics.

However, the frequency modulator using the above-described third methodhas to be supplied with an analog modulation signal, which results inthe need of a high-precision D/A converter that converts digitalmodulation data into the analog modulation signal. Furthermore, themodulation signal is supplied to both the reference signal source andthe voltage controlled oscillator, which results in the need ofindividual adjustment of the level of the modulation signal in both thereference signal source and the voltage controlled oscillator in orderto obtain good frequency modulation characteristics.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a frequencymodulator that can use a reference signal source having no frequencymodulation function, and perform modulation over a wide range offrequencies based on a digital modulation signal.

The present invention has the following features to attain the objectmentioned above.

A first aspect of the present invention is directed to a frequencymodulator that performs frequency modulation based on digital modulationdata, including: a sigma-delta modulator for sigma-delta modulating dataobtained by adding a fractional part of provided frequency divisionfactor data with modulation data; a voltage controlled oscillator thatcan control an oscillation frequency; a variable frequency divider fordividing a frequency of an output of the voltage controlled oscillatorbased on data obtained by adding an output of the sigma-delta modulatorwith an integral part of the frequency division factor data; a phasecomparator for comparing phases of a provided reference signal and anoutput of the variable frequency divider; and a loop filter forsmoothing an output of the phase comparator and providing the smoothedsignal to the voltage controlled oscillator, wherein the output of thevoltage controlled oscillator is frequency-modulated based on a controlsignal that is an analog signal converted from the output of thesigma-delta modulator.

Thus, according to the first aspect, frequency modulation by thevariable frequency divider and frequency modulation by the voltagecontrolled oscillator are concurrently performed based on the digitalmodulation data. The degree of modulation of the output signalcorresponds to the sum of the modulation characteristics of theabove-described two types of frequency modulation, thereby frequencymodulation characteristics that are flat over a bandwidth wider than aloop bandwidth are obtained. Thus, it is possible to use a referencesignal source having no frequency modulation function, and performfrequency modulation over a wide range of frequencies based on a digitalmodulation signal.

In this case, the voltage controlled oscillator may include a frequencycontrol terminal connected to the loop filter and a frequency modulationterminal for inputting the control signal, and the voltage controlledoscillator may control the oscillation frequency based on an output ofthe loop filter and the control signal inputted from the frequencymodulation terminal. As a result, it is possible to perform frequencymodulation over a wide range of frequencies based on the digitalmodulation data by using the voltage controlled oscillator having thefrequency modulation terminal. Furthermore, the voltage controlledoscillator whose change of output frequency in response to a change ininput frequency at the frequency modulation terminal may be smallercompared to its change of output frequency in response to a change ininput frequency at the frequency control terminal, thereby enabling toincrease the modulation accuracy.

Alternatively, the control signal may be connected to a junction pointbetween the loop filter and the voltage controlled oscillator. Thus, itis possible to perform frequency modulation over a wide range offrequencies based on the digital modulation data by using the voltagecontrolled oscillator having no frequency modulation terminal.Alternatively, a frequency bandwidth of the modulation data may be widerthan a bandwidth of the loop filter, and a highest frequency of themodulation data may be smaller than a frequency of the reference signal.

Alternatively, the frequency modulator may further include, on a routefrom the sigma-delta modulator to the voltage controlled oscillator, anamplitude adjustment circuit for adjusting signal amplitude. Thus, it ispossible to obtain a correct output signal by adjusting the amplitude ofthe control signal in accordance with characteristics of the voltagecontrolled oscillator. Alternatively, the frequency modulator mayfurther include, on a route from the sigma-delta modulator to thevoltage controlled oscillator, a low-pass filter for removing a noisecomponent caused in the sigma-delta modulator. Thus, it is possible toobtain an output signal with low-noise components by removing the noisecomponent caused in the sigma-delta modulator.

Alternatively, the sigma-delta modulator may operate by using either anoutput of the variable frequency divider or the reference signal as aclock, and may be of at least a second order or higher. Thus, it ispossible to perform sigma-delta modulation without the need to generatean additional high-speed clock signal, and also obtain a stable outputsignal by performing highly stable sigma-delta modulation.

A second aspect of the present invention is directed to a frequencymodulator that performs frequency modulation based on digital modulationdata, including: a sigma-delta modulator for sigma-delta modulating dataobtained by adding a fractional part of provided frequency divisionfactor data with modulation data; a voltage controlled oscillator thatcan control an oscillation frequency; a variable frequency divider fordividing a frequency of an output of the voltage controlled oscillatorbased on data obtained by adding an output of the sigma-delta modulatorwith an integral part of the frequency division factor data; a phasecomparator for comparing phases of a provided reference signal and anoutput of the variable frequency divider; and a loop filter forsmoothing an output of the phase comparator and providing the smoothedsignal to the voltage controlled oscillator, wherein the output of thevoltage controlled oscillator is frequency-modulated based on a controlsignal that is an analog signal converted from data obtained by addingthe output of the sigma-delta modulator with the integral part of thefrequency division factor data. According to the above-described secondaspect, the same effect can be produced as in the case of the firstaspect.

A third aspect of the present invention is directed to a frequencymodulator that performs frequency modulation based on digital modulationdata, including: a sigma-delta modulator for sigma-delta modulating dataobtained by adding a fractional part of provided reference frequencydivision factor data with modulation data; a reference frequency dividerfor dividing a frequency of a provided reference signal based on dataobtained by adding an output of the sigma-delta modulator with anintegral part of the reference frequency division factor data; a voltagecontrolled oscillator that can control an oscillation frequency; afrequency divider for dividing a frequency of an output of the voltagecontrolled oscillator; a phase comparator for comparing phases of thereference signal frequency-divided by the reference frequency dividerand an output of the frequency divider; and a loop filter for smoothingan output of the phase comparator and providing the smoothed signal tothe voltage controlled oscillator, wherein the output of the voltagecontrolled oscillator is frequency-modulated based on a control signalthat is an analog signal converted from the output of the sigma-deltamodulator.

Thus, according to the third aspect, frequency modulation by thereference signal source and frequency modulation by the voltagecontrolled oscillator are concurrently performed based on the digitalmodulation data. The degree of modulation of the output signalcorresponds to the sum of the modulation characteristics of theabove-described two types of frequency modulation, thereby frequencymodulation characteristics that are flat over a bandwidth wider than aloop bandwidth are obtained. Thus, it is possible to use a referencesignal source having no frequency modulation function, and performfrequency modulation over a wide range of frequencies based on a digitalmodulation signal.

In this case, the frequency divider may be a variable frequency divider,thereby being able to handle varying frequencies. Alternatively, thevoltage controlled oscillator may include a frequency control terminalconnected to the loop filter and a frequency modulation terminal forinputting the control signal, and the voltage controlled oscillator maycontrol the oscillation frequency based on an output of the loop filterand the control signal inputted from the frequency modulation terminal.Thus, it is possible to perform frequency modulation over a wide rangeof frequencies based on the digital modulation data by using the voltagecontrolled oscillator having the frequency modulation terminal.Furthermore, the voltage controlled oscillator whose change of outputfrequency in response to a change in input frequency at the frequencymodulation terminal may be smaller compared to its change of outputfrequency in response to a change in input frequency at the frequencycontrol terminal, thereby enabling to increase the modulation accuracy.

Alternatively, the control signal may be connected to a junction pointbetween the loop filter and the voltage controlled oscillator. Thus, itis possible to perform frequency modulation over a wide range offrequencies based on the digital modulation data by using the voltagecontrolled oscillator having no frequency modulation terminal.Alternatively, a frequency bandwidth of the modulation data may be widerthan a bandwidth of the loop filter, and a highest frequency of themodulation data may be smaller than a frequency of the reference signal.

Alternatively, the frequency modulator may further include, on a routefrom the sigma-delta modulator to the voltage controlled oscillator, anamplitude adjustment circuit for adjusting signal amplitude. Thus, it ispossible to obtain a correct output signal by adjusting the amplitude ofthe control signal in accordance with characteristics of the voltagecontrolled oscillator. Alternatively, the frequency modulator mayfurther include, on a route from the sigma-delta modulator to thevoltage controlled-oscillator, a low-pass filter for removing a noisecomponent caused in the sigma-delta modulator. Thus, it is possible toobtain an output signal with low-noise components by removing the noisecomponent caused in the sigma-delta modulator.

Alternatively, the sigma-delta modulator may operate by using thereference signal as a clock, and may be of at least a second order orhigher. Thus, it is possible to perform sigma-delta modulation withoutthe need to generate an additional high-speed clock signal, and alsoobtain a stable output signal by performing highly stable sigma-deltamodulation.

A fourth aspect of the present invention is directed to a frequencymodulator that performs frequency modulation based on digital modulationdata, including: a sigma-delta modulator for sigma-delta modulating dataobtained by adding a fractional part of provided reference frequencydivision factor data with modulation data; a reference frequency dividerfor dividing a frequency of a provided reference signal based on dataobtained by adding an output of the sigma-delta modulator with anintegral part of the reference frequency division factor data; a voltagecontrolled oscillator that can control an oscillation frequency; afrequency divider for dividing a frequency of an output of the voltagecontrolled oscillator; a phase comparator for comparing phases of thereference signal frequency-divided by the reference frequency dividerand an output of the frequency divider; and a loop filter for smoothingan output of the phase comparator and providing the smoothed signal tothe voltage controlled oscillator, wherein the output of the voltagecontrolled oscillator is frequency-modulated based on a control signalthat is an analog signal converted from data obtained by adding theoutput of the sigma-delta modulator with the integral part of thereference frequency division factor data. According to theabove-described fourth aspect, the same effect can be produced as in thecase of the third aspect.

A fifth aspect of the present invention is directed to a frequencymodulating method for performing frequency modulation based on digitalmodulation data by using a phase locked loop having a voltage controlledoscillator, a variable frequency divider, a phase comparator, and a loopfilter, including; a step of sigma-delta modulating data obtained byadding a fractional part of provided frequency division factor data withmodulation data; a step of providing data obtained by adding thesigma-delta modulated signal with an integral part of the frequencydivision factor data to the variable frequency divider as effectivefrequency division factor data; and a step of frequency modulating anoutput of the voltage controlled oscillator based on a control signalthat is an analog signal converted from the sigma-delta modulatedsignal.

A sixth aspect of the present invention is directed to a frequencymodulating method for performing frequency modulation based on digitalmodulation data by using a phase locked loop having a voltage controlledoscillator, a frequency divider, a phase comparator, and a loop filter,including; a step of sigma-delta modulating data obtained by adding afractional part of provided reference frequency division factor datawith modulation data; a step of dividing a frequency of a providedreference signal based on data obtained by adding the sigma-deltamodulated signal with an integral part of the reference frequencydivision factor data and providing the frequency-divided signal to thephase locked loop; and a step of frequency modulating an output of thevoltage controlled oscillator based on a control signal that is ananalog signal converted from the sigma-delta modulated signal.

Thus, according to either the above-described fifth or sixth aspect,frequency modulation by the variable frequency divider (or a referencesignal source) and frequency modulation by the voltage controlledoscillator are concurrently performed based on the digital modulationdata. The degree of modulation of the output signal corresponds to thesum of the modulation characteristics of the above-described two typesof frequency modulation, thereby frequency modulation characteristicsthat are flat over a bandwidth wider than a loop bandwidth are obtained.Thus, it is possible to use a reference signal source having nofrequency modulation function, and perform frequency modulation over awide range of frequencies based on a digital modulation signal.

A seventh aspect of the present invention is directed to a wirelesscircuit that transmits and receives a signal frequency-modulated basedon digital modulation data, including: a reference oscillator forgenerating a reference signal; a frequency modulator that operates basedon the reference signal; an antenna for transmitting and receiving anelectric wave; a transmission amplifier for amplifying an output of thefrequency modulator and outputting the amplified signal to the antenna;and a receiving circuit for processing a signal received by the antenna,wherein the frequency modulator includes: a sigma-delta modulator forsigma-delta modulating data obtained by adding a fractional part ofprovided frequency division factor data with modulation data; a voltagecontrolled oscillator that can control an oscillation frequency; avariable frequency divider for dividing a frequency of an output of thevoltage controlled oscillator based on data obtained by adding an outputof the sigma-delta modulator with an integral part of the frequencydivision factor data; a phase comparator for comparing phases of thereference signal and an output of the variable frequency divider; and aloop filter for smoothing an output of the phase comparator andproviding the smoothed signal to the voltage controlled oscillator,wherein the output of the voltage controlled oscillator isfrequency-modulated based on a control signal that is an analog signalconverted from the output of the sigma-delta modulator, wherein whendata is transmitted, the frequency modulator is controlled so as toperform frequency modulation based on data to be transmitted, and whendata is received, the frequency modulator is controlled so as to outputa non-modulated signal, and wherein the receiving circuit processes thesignal received by the antenna by using the non-modulated signaloutputted from the frequency modulator as a local signal.

An eighth aspect of the present invention is directed to a wirelesscircuit that transmits and receives a signal frequency-modulated basedon digital modulation data, including: a reference oscillator forgenerating a reference signal; a frequency modulator that operates basedon the reference signal; an antenna for transmitting and receiving anelectric wave; a transmission amplifier for amplifying an output of thefrequency modulator and outputting the amplified signal to the antenna;and a receiving circuit for processing a signal received by the antenna,wherein the frequency modulator includes: a sigma-delta modulator forsigma-delta modulating data obtained by adding a fractional part ofprovided reference frequency division factor data with modulation data;a reference frequency divider for dividing a frequency of the referencesignal based on data obtained by adding an output of the sigma-deltamodulator with an integral part of the reference frequency divisionfactor data; a voltage controlled oscillator that can control anoscillation frequency; a frequency divider for dividing a frequency ofan output of the voltage controlled oscillator; a phase comparator forcomparing phases of the reference signal frequency-divided by thereference frequency divider and an output of the frequency divider; anda loop filter for smoothing an output of the phase comparator andproviding the smoothed signal to the voltage controlled oscillator, andwherein the output of the voltage controlled oscillator isfrequency-modulated based on a control signal that is an analog signalconverted from the output of the sigma-delta modulator, wherein whendata is transmitted, the frequency modulator is controlled so as toperform frequency modulation based on data to be transmitted, and whendata is received, the frequency modulator is controlled so as to outputa non-modulated signal, and wherein the receiving circuit processes thesignal received by the antenna by using the non-modulated signaloutputted from the frequency modulator as a local signal.

Thus, according to either the above-described seventh or eighth aspect,even if no analog modulation signal is provided, it is possible torealize a wireless circuit by providing digital data for designating afrequency channel and digital modulation data for performing frequencymodulation.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the structure of a frequencymodulator according to a first embodiment of the present invention;

FIG. 2 is an exemplary illustration of the structure of a sigma-deltamodulator in the frequency modulator according to the first embodimentand a second embodiment of the present invention;

FIG. 3 is an exemplary illustration of another structure of thesigma-delta modulator in the frequency modulator according to the firstand second embodiments;

FIG. 4 is an exemplary illustration of another structure of thesigma-delta modulator in the frequency modulator according to the firstand second embodiments;

FIG. 5 is an exemplary illustration of another structure of thesigma-delta modulator in the frequency modulator according to the firstand second embodiments;

FIG. 6 is an illustration showing an example of quantization noisecharacteristics in the sigma-delta modulator;

FIG. 7 is a block diagram illustrating the structure of a frequencymodulator according to a variant of the first embodiment of the presentinvention;

FIG. 8 is a block diagram illustrating the structure of a frequencymodulator according to another variant of the first embodiment of thepresent invention;

FIG. 9 is a block diagram illustrating the structure of the frequencymodulator according to the second embodiment of the present invention;

FIG. 10 is a block diagram illustrating the structure of a frequencymodulator according to a variant of the second embodiment of the presentinvention;

FIG. 11 is a block diagram illustrating the structure of a frequencymodulator according to another variant of the second embodiment of thepresent invention;

FIG. 12 is a block diagram illustrating the structure of a wirelesscircuit according to a third embodiment of the present invention;

FIG. 13 is a block diagram illustrating the structure of a conventionalfrequency modulator; and

FIG. 14 is an illustration showing frequency modulation characteristicsof the frequency modulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1 is a block diagram illustrating the structure of a frequencymodulator according to a first embodiment of the present invention. Thefrequency modulator shown in FIG. 1 includes a voltage controlledoscillator 1, a variable frequency divider 2, a phase comparator 3, aloop filter 4, a sigma-delta modulator 5, a D/A converter 6, a low-passfilter 7, an amplitude adjustment circuit 8, and adders 11 and 12. Areference signal with a predetermined frequency is provided to thefrequency modulator from an external source (not shown), which can be asignal source having no frequency modulation function.

The voltage controlled oscillator 1, the variable frequency divider 2,the phase comparator 3, and the loop filter 4 form a phase locked loop(PLL) described below. The variable frequency divider 2 divides afrequency of an output signal of the voltage controlled oscillator 1based on provided effective frequency division factor data 13. The phasecomparator 3 compares the phases of the output signal of the variablefrequency divider 2 and the reference signal. The output signal of thephase comparator 3 is inputted into a frequency control terminal of thevoltage controlled oscillator 1 after passing through the loop filter 4.The voltage controlled oscillator 1 oscillates at a frequencyappropriate to the output signal of the loop filter 4. This PLL performsfeedback control so as to keep a center frequency of the output signalof the voltage controlled oscillator 1 at a predetermined value.

The effective frequency division factor data 13 provided to the variablefrequency divider 2 is calculated as follows. Frequency division factordata is externally provided to the variable frequency divider 2 as a setof an integral part M1 of the frequency division factor data and afractional part M2 of the frequency division factor data. The adder 11adds the fractional part M2 with digital modulation data X. Thesigma-delta modulator 5 sigma-delta modulates the output data of theadder 11 by using the output signal of the variable frequency divider 2as a clock. The output signal of the sigma-delta modulator 5 is inputtedinto the adder 12 and the D/A converter 6. The adder 12 adds the outputsignal of the sigma-delta modulator 5 with the integral part M1. Theoutput data of the adder 12 becomes the effective frequency divisionfactor data 13.

On the other hand, the D/A converter 6 converts the output signal of thesigma-delta modulator 5 into an analog signal. The analog signal becomesa control signal 14 after passing through the low-pass filter 7 and theamplitude adjustment circuit 8. As the low-pass filter 7, a filterhaving a passband wider than the bandwidth of the modulation signal isused. The low-pass filter 7 removes a higher frequency noise componentcaused in the sigma-delta modulator 5 from the analog signal outputtedfrom the D/A converter 6. The amplitude adjustment circuit 8 adjusts themagnitude of the output signal of the low-pass filter 7 so as to be avalue appropriate to the modulation sensitivity of the frequencymodulation terminal of the voltage controlled oscillator 1. The controlsignal 14 is inputted into the frequency modulation terminal of thevoltage controlled oscillator 1.

The frequency modulator structured as above performs two types offrequency modulation at the same time. In a first frequency modulation,the modulation data X is sigma-delta modulated. The sigma-deltamodulated data is then provided to the variable frequency divider 2 asthe effective frequency division factor data 13. Thus, the output signalof the PLL is frequency-modulated. The first frequency modulationproduces an effect similar to the low-pass characteristics shown in FIG.14 in solid line. In a second frequency modulation, the modulation dataX becomes a signal changing in a stepwise manner due to the actions ofthe sigma-delta modulator 5 and the D/A converter 6. The signal becomesthe control signal 14 after passing through the low-pass filter 7 andthe amplitude adjustment circuit 8. The control signal 14 is inputtedinto the frequency modulation terminal of the voltage controlledoscillator 1. Thus, the output signal of the voltage controlledoscillator 1 is frequency-modulated. The second frequency modulationproduces an effect similar to the high-pass characteristics shown inFIG. 14 in dashed line.

Therefore, the degree of modulation of the output signal of thefrequency modulator corresponds to the sum of the low-passcharacteristics (solid line) and the high-pass characteristics (dashedline) shown in FIG. 14. Thus, the frequency modulator according to thepresent embodiment enables the use of a reference signal source havingno frequency modulation function, and also allows flat frequencymodulation characteristics to be obtained for a modulation signal havinga bandwidth wider than a loop bandwidth based on digital modulationdata. Especially, as shown in FIG. 14, even if the frequency bandwidthof the modulation data is wider than the bandwidth of the loop filter,and the highest frequency of the modulation data is smaller than thefrequency of the reference signal, the same effect can be produced.

The second frequency modulation described above is performed for thefollowing reason. The frequency modulator according to the presentembodiment is provided with high-precision digital modulation data X asa modulation signal. The modulation data X is treated as a variant ofthe fractional part M2 of the frequency division factor data. That is,the modulation data X is added to the fractional part M2, andsigma-delta modulated by the sigma-delta modulator 5. Based on theinputted data, the sigma-delta modulator 5 outputs data provided with asmaller number of bits but with a higher clock frequency than theinputted data. As a result, if the clock of the sigma-delta modulator 5is set sufficiently high compared with frequency components of themodulation signal, the sigma-delta modulated data includes almost allinformation included in the modulation signal.

Quantization noise caused by sigma-delta modulation is added to thesigma-delta modulated data. Thus, the quantization noise is alsointroduced in the output signal of the variable frequency divider 2. Thequantization noise is sufficiently reduced at the loop filter 4 that haslow-pass characteristics, thereby being prevented from being introducedinto the output signal of the voltage controlled oscillator 1. However,among frequency components of the modulation signal, frequencycomponents higher than the loop bandwidth (determined mainly by thecharacteristics of the loop filter 4) of the PLL are attenuated at thesame time when the signal passes through the loop filter 4. In order tocompensate for the above-described attenuated frequency components, thefrequency modulator according to the present embodiment provides thecontrol signal 14, which is an analog signal converted from thesigma-delta modulated data, to the frequency modulation terminal of thevoltage controlled oscillator 1. This is the reason why the secondfrequency modulation is performed.

The frequency modulator used in digital communications, etc., isprovided with high-precision digital modulation data X. Therefore, it isnecessary to use a high-precision D/A converter to directly D/A convertthe modulation data X. On the other hand, the output of the sigma-deltamodulator 5 according to the present embodiment is a signal with ahigh-speed clock signal but with a smaller number of bits, therebyallowing the use of the simple and moderate-precision D/A converter 6.Even with such a D/A converter 6, it is possible to reproduce ahigh-precision analog frequency modulation signal, and provide thereproduced signal to the frequency modulation terminal of the voltagecontrolled oscillator 1 by removing a higher frequency noise componentcaused by sigma-delta modulation by the low-pass filter 7.

Hereinafter, the sigma-delta modulator 5 in the frequency modulator ofthe present embodiment is described. FIGS. 2 to 5 are all exemplaryillustrations of the structure of the sigma-delta modulator 5 using a Ztransform, wherein one clock delay is denoted as Z⁻¹.

FIG. 2 is an exemplary illustration of the structure of a second-ordersigma-delta modulator. The sigma-delta modulator 5 a shown in FIG. 2includes adders 101, 102, 104, and 105, delay circuits 103, 106, and108, a quantizer 107, and a multiplier 109. The quantizer 107 quantizesan output of the adder 105 by a quantization value L. The output of thequantizer 107 is inputted into the multiplier 109 after passing throughthe delay circuit 108. The multiplier 109 multiplies the input by thequantization value L. The output of the multiplier 109 is inputted intothe adders 101 and 104. The adder 101 subtracts the output of themultiplier 109 from an input F of the sigma-delta modulator 5 a. Theoutput of the adder 101 is inputted into the adder 104 after passingthrough a first-order integrator structured by the adder 102 and thedelay circuit 103. From this input, the adder 104 subtracts the outputof the multiplier 109. The output of the adder 104 is inputted into thequantizer 107 after passing through a first-order integrator structuredby the adder 105 and the delay circuit 106.

In the sigma-delta modulator 5 a, a relationship between the input F andan output Y is represented as Y=F/L+(1−Z⁻¹)²×Q. Thus, if a clockfrequency is denoted by f_(s), frequency characteristics for |1−Z⁻¹| aregiven as |2 sin(πf/f_(s))|. As a result, frequency characteristic |2 sin(πf/f_(s))|² is multiplied with the quantization noise Q in thesigma-delta modulator 5 a.

FIG. 3 is an exemplary illustration of the structure of a third-ordersigma-delta modulator. The sigma-delta modulator 5 b shown in FIG. 3includes first-order sigma-delta modulators 121, 122, and 123, afirst-order differentiator 124, a second-order differentiator 125, andadders 126 and 127. The sigma-delta modulator 5 b has a multistagestructure with a plurality of the first-order sigma-delta modulatorsconnected to each other. That is, a value corresponding to a fractionalpart of the first-order sigma-delta modulator 121 is inputted into thefirst-order sigma-delta modulator 122, and a value corresponding to afractional part of the first-order sigma-delta modulator 122 is inputtedinto the first-order sigma-delta modulator 123. Furthermore, a valuecorresponding to an integral part of the first-order sigma-deltamodulator 122 is inputted into the first-order differentiator 124, and avalue corresponding to an integral part of the first-order sigma-deltamodulator 123 is inputted into the second-order differentiator 125. Theadders 126 and 127 add the value corresponding to the integral part ofthe first-order sigma-delta modulator 121, the output of the first-orderdifferentiator 124, and the output of the second-order differentiator125. As a result, an output Y of the sigma-delta modulator 5 b iscalculated.

In the sigma-delta modulator 5 b, a relationship between an input F andthe output Y is represented as Y=F/L+(1−Z⁻¹)³×Q. Therefore, according tothe same consideration as in the case of the sigma-delta modulator 5 a,frequency characteristic |2 sin (πf/f_(s))|³ (f_(s): clock frequency) ismultiplied with the quantization noise Q in the sigma-delta modulator 5b.

FIG. 4 is an exemplary illustration of the structure of a fourth-ordersigma-delta modulator. The sigma-delta modulator 5 c shown in FIG. 4uses two second-order sigma-delta modulators differently structured fromthe sigma-delta modulator 5 a shown in FIG. 2. The sigma-delta modulator5 c includes a first second-order sigma-delta modulator 141, a secondsecond-order sigma-delta modulator 142, a delay circuit 143, asecond-order differentiator 144, a multiplier 145, and adders 146 and147.

An input F of the sigma-delta modulator 5 c is inputted into the firstsecond-order sigma-delta modulator 141. A value corresponding to afractional part of the first second-order sigma-delta modulator 141 isinputted into the second second-order sigma-delta modulator 142 afterpassing through the multiplier 145 and the adder 146. A valuecorresponding to an integral part of the first second-order sigma-deltamodulator 141 is delayed by 1 clock period by the delay circuit 143, andinputted into the adder 147. The output of the second second-ordersigma-delta modulator 142 is inputted into the adder 147 after passingthrough the second-order differentiator 144. The adder 147 adds thesetwo signals, thereby calculating an output Y of the sigma-deltamodulator 5 c.

In the sigma-delta modulator 5 c, a relationship between the input F andthe output Y is represented as Y=−Z⁻²×F/L+(1−Z⁻¹)⁴×Q. Therefore,according to the same consideration as in the case of the sigma-deltamodulators 5 a and 5 b, frequency characteristic |2 sin (πf/f_(s))|⁴(f_(s): clock frequency) is multiplexed with the quantization noise Q inthe sigma-delta modulator 5 c.

FIG. 5 is an exemplary illustration of the structure of a fifth-ordersigma-delta modulator. The sigma-delta modulator 5 d shown in FIG. 5includes first-order integrators 161 to 165, adders 166 to 169,coefficient multipliers 171 to 177, a quantizer 178, and a multiplier179.

The quantizer 178 quantizes an output of the adder 169 by thequantization value L. The multiplier 179 multiplies the output of thequantizer 178 by the quantization value L. The output of the multiplier179 is inputted into the adder 166. The adder 166 subtracts the outputof the multiplier 179 from an input F of the sigma-delta modulator 5 d.The first-order integrator 161 integrates the output of the adder 166.The adder 167 adds the output of the first-order integrator 161 and theoutput of the coefficient multiplier 176. The first-order integrator 162integrates the output of the adder 167, and the first-order integrator163 integrates the output of the first-order integrator 162. Thecoefficient multiplier 176 multiplies the output of the first-orderintegrator 163 by a predetermined coefficient (in FIG. 5, −a₁). Theadder 168 adds the output of the first-order integrator 163 with theoutput of the coefficient multiplier 177. The first-order integrator 164integrates the output of the adder 168, and the first-order integrator165 integrates the output of the first-order integrator 164. Thecoefficient multiplier 177 multiplies the output of the first-orderintegrator 165 by a predetermined coefficient (in FIG. 5, −a₂).

The coefficient multipliers 171 to 175 each multiply the outputs of thefirst-order integrators 161 to 165 by predetermined coefficients (inFIG. 5, c₁ to c₅). The adder 169 sums the outputs of the coefficientmultipliers 171 to 175. The output of the quantizer 178 becomes anoutput Y of the sigma-delta modulator 5 d. The sigma-delta modulator 5 dallows frequency characteristics of a fifth-order sigma-delta modulationto be changed arbitrarily by arbitrarily setting a coefficient of eachcoefficient multiplier.

Four exemplary structures of the sigma-delta modulator 5 have been shownas described above, but these circuits are illustrative only. In otherwords, an arbitrary sigma-delta modulator may be used as the sigma-deltamodulator 5 with consideration given to the size of a circuit, frequencycharacteristics, and delay characteristics, etc. For example, as thesigma-delta modulator 5, second to fifth-order sigma-delta modulatorsdifferently structured from those described above may be used, or ahigher order sigma-delta modulator may be used.

In order to design the sigma-delta modulator 5, it is necessary to firstdetermine the order of the modulator. FIG. 6 is an illustration showingan example of quantization noise characteristics in second tofifth-order sigma-delta modulators. In FIG. 6, the horizontal axisrepresents a normalized frequency in a log scale, and the vertical axisrepresents quantization noise in decibels (dB). As shown in FIG. 6, thenoise level around direct current (DC) is reduced as the order of thesigma-delta modulator becomes higher. With consideration given to such apoint as described above, the sigma-delta modulator 5 may be designed.

Various structures described as follows can be taken as variants of thefrequency modulator according to the present embodiment. First, in thefrequency modulator shown in FIG. 1, the control signal 14 is assumed tobe inputted into the frequency modulation terminal of the voltagecontrolled oscillator 1. However, in the case where a voltage controlledoscillator 1 having no frequency modulation terminal is used, thecontrol signal 14 may be connected to a junction point between the loopfilter 4 and the voltage controlled oscillator 1 as shown in FIG. 7. Thefrequency modulator shown in FIG. 7 operates in the same manner andproduces the same effect as is the case with the frequency modulatorshown in FIG. 1.

Furthermore, in the frequency modulator shown in FIG. 1, the output ofthe sigma-delta modulator 5 is assumed to become the control signal 14after passing through the D/A converter 6, the low-pass filter 7, andthe amplitude adjustment circuit 8. Otherwise, as shown in FIG. 8, theoutput of the adder 12 (that is, data obtained by adding the output ofthe sigma-delta modulator 5 and with an integral part M1 of thefrequency division factor data) may become the control signal 14 afterpassing through the D/A converter 6, the low-pass filter 7, and theamplitude adjustment circuit 8. If the amplitude adjustment circuit 8,which can process an input signal to which the integral part M1 has beenadded, is used, the frequency modulator shown in FIG. 8 operates in thesame manner and produces the same effect as is the case with thefrequency modulator shown in FIG. 1.

Still further, the sigma-delta modulator 5 may use, as a clock, areference signal provided from an external source. Also, it is possibleto increase the modulation accuracy by using a voltage controlledoscillator 1 whose change of output frequency in response to a change ininput frequency at the frequency modulation terminal is smaller comparedto its change of output frequency in response to a change in inputfrequency at the frequency control terminal.

As long as the D/A converter 6, the low-pass filter 7, and the amplitudeadjustment circuit 8 are placed on the route from the sigma-deltamodulator 5 to the voltage controlled oscillator 1, a connecting orderthereof may be arbitrary. Also, if the frequency modulation sensitivityof the voltage controlled oscillator 1 is constant, the amplitudeadjustment circuit 8 may be a fixed attenuator. Furthermore, the voltagecontrolled oscillator 1 may be provided with the frequency modulationterminal that also functions as a terminal connected to the loop filter4, or may be provided with the frequency modulation terminal separatelyfrom the terminal connected to the loop filter 4. Still further, a partof the components shown in FIG. 1 (for example, the sigma-deltamodulator 5) may be realized by a program.

Second Embodiment

FIG. 9 is a block diagram illustrating the structure of the frequencymodulator according to a second embodiment of the present invention. Thefrequency modulator shown in FIG. 9 includes the voltage controlledoscillator 1, the variable frequency divider 2, the phase comparator 3,the loop filter 4, the sigma-delta modulator 5, the D/A converter 6, thelow-pass filter 7, the amplitude adjustment circuit 8, a referencefrequency divider 9, and the adders 11 and 12. Any components thatfunction in similar manners to their counterparts in the firstembodiment are denoted by like numerals, with the descriptions thereofomitted.

As is the case with the first embodiment, a reference signal with apredetermined frequency is provided to the frequency modulator shown inFIG. 9 from an external signal source (not shown), which can be a signalsource having no frequency modulation function. The voltage controlledoscillator 1, the variable frequency divider 2, the phase comparator 3,and the loop filter 4 form a phase locked loop (PLL), as is the casewith the first embodiment. However, the PLL shown in FIG. 9 differs fromthat shown in FIG. 1 in the following four points. First, the referencesignal provided from the external source is frequency-divided by thereference frequency divider 9, and inputted into the PLL. Second, thevariable frequency divider 2 divides a frequency of an output signal ofthe voltage controlled oscillator 1 in accordance with frequencydivision factor data independent of the modulation data X. Third, thephase comparator 3 compares the phases of an output signal of thevariable frequency divider 2 and an output signal of the referencefrequency divider 9. Fourth, the variable frequency divider 2 may be afrequency divider whose frequency division factor is fixed.

The reference frequency divider 9 divides a frequency of the externallyprovided reference signal in accordance with provided effectivereference frequency division factor data 15. The effective referencefrequency division factor data 15 provided to the reference frequencydivider 9 is calculated by substantially the same method as that usedfor calculating the effective frequency division factor data 13according to the first embodiment. That is, reference frequency divisionfactor data is externally provided to the reference frequency divider 9as a set of an integral part N1 of the reference frequency divisionfactor data and a fractional part N2 of the reference frequency divisionfactor data. The adder 11 adds the fractional part N2 with digitalmodulation data X. The sigma-delta modulator 5 sigma-delta modulates theoutput data of the adder 11 by using the reference signal as a clock.The adder 12 adds the output signal of the sigma-delta modulator 5 withthe integral part N1. The sum obtained by the adder 12 becomes theeffective reference frequency division factor data 15.

On the other hand, the output signal of the sigma-delta modulator 5becomes the control data 14 after passing through the D/A converter 6,the low-pass filter 7, and the amplitude adjustment circuit 8, as is thecase with the first embodiment. The control signal 14 is inputted intothe frequency modulation terminal of the voltage controlled oscillator1.

The frequency modulator structured as above performs two types offrequency modulation at the same time, as is the case with the firstembodiment. In a first frequency modulation, the modulation data X issigma-delta modulated, and provided to the reference frequency divider 9as the effective reference frequency division factor data 15. As aresult, the output signal of the PLL is frequency-modulated. The firstfrequency modulation produces an effect similar to the low-passcharacteristics shown in FIG. 14 in solid line. A second frequencymodulation is the same as that described in the first embodiment. Thesecond frequency modulation produces an effect similar to the high-passcharacteristics shown in FIG. 14 in dashed line.

Therefore, the degree of modulation of the output signal of thefrequency modulator corresponds to the sum of the low-passcharacteristics (solid line) and the high-pass characteristics (dashedline) shown in FIG. 14. Thus, the frequency modulator according to thepresent embodiment enables the use of a reference signal source havingno frequency modulation function, and also allows flat frequencymodulation characteristics to be obtained for a modulation signal havinga bandwidth wider than a loop bandwidth based on digital modulationdata. Especially, as shown in FIG. 14, even if the frequency bandwidthof the modulation data is wider than the bandwidth of the loop filter,and the highest frequency of the modulation data is smaller than thefrequency of the reference signal, the same effect can be produced.

Note that the structure of the frequency modulator according to thepresent embodiment can take the same variants (except for clockselectability of the sigma-delta modulator 5) described in the firstembodiment. For example, as the sigma-delta modulator 5, the sigma-deltamodulators 5 a to 5 d shown in FIGS. 2 to 5, respectively, second tofifth-order sigma-delta modulators structured differently from thoseshown in FIGS. 2 to 5, or a higher order sigma-delta modulator may beused. Furthermore, as shown in FIG. 10, the control signal 14 may beconnected to a junction point between the loop filter 4 and the voltagecontrolled oscillator 1. Still further, as shown in FIG. 11, an outputof the adder 12 (that is, data obtained by adding an output of thesigma-delta modulator 5 with the integral part M1 of the frequencydivision factor data) may become the control signal 14 after passingthrough the D/A converter 6, the low-pass filter 7, and the amplitudeadjustment circuit 8.

Third Embodiment

FIG. 12 is a block diagram illustrating the structure of a wirelesscircuit according to a third embodiment of the present invention. Thewireless circuit according to the present embodiment uses the frequencymodulator according to the first or second embodiment. The wirelesscircuit shown in FIG. 12 includes a reference oscillator 201, afrequency modulator 202, a transmission amplifier 203, a receivingcircuit 204, a duplexer 205, and an antenna 206. This wireless circuittransmits and receives data to and from a wireless circuit at the otherend (not shown).

The reference oscillator 201 generates a reference signal with apredetermined frequency. The reference oscillator 201 is structured by,for example, a crystal oscillator whose operation is stable. Thereference signal generated by the reference oscillator 201 is providedto the frequency modulator 202. The frequency modulator 202 is afrequency modulator according to the first or second embodiment, andoperates based on the reference signal generated by the referenceoscillator 201.

The output signal of the frequency modulator 202 is used differently indata transmission and data reception. When data is transmitted (in FIG.12, shown as a solid line), the data to be transmitted is inputted intothe frequency modulator 202 as modulation data, and the output signal ofthe frequency modulator 202 is inputted into the transmission amplifier203 as a frequency-modulated wave. The transmission amplifier 203amplifies the inputted frequency-modulated wave. The signal amplified bythe transmission amplifier 203 is transmitted from the antenna 206 as anelectric wave after passing through the duplexer 205.

When data is received (in FIG. 12, shown as a dashed line), nomodulation data is inputted into the frequency modulator 202. Otherwise,the frequency modulator 202 is controlled so as to output anon-modulated signal. The non-modulated signal outputted from thefrequency modulator 202 is inputted into the receiving circuit 204 as alocal signal for demodulating a wireless signal, and also inputted intothe duplexer 205. The receiving circuit 204 uses the non-modulatedsignal as the local signal, and demodulates the output signal of theduplexer 205.

As described above, even if no analog modulation signal is provided, thewireless circuit according to the present embodiment can realize awireless circuit by providing digital data for designating a frequencychannel and digital modulation data for performing frequency modulation.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A frequency modulator that performs frequency modulation based ondigital modulation data, comprising: a sigma-delta modulator forsigma-delta modulating data obtained by adding a fractional part ofprovided reference frequency division factor data with modulation data;a reference frequency divider for dividing a frequency of a providedreference signal based on data obtained by adding an output of thesigma-delta modulator with an integral part of the reference frequencydivision factor data; a voltage controlled oscillator operable tocontrol an oscillation frequency; a frequency divider for dividing afrequency of an output of the voltage controlled oscillator; a phasecomparator for comparing phases of the reference signalfrequency-divided by the reference frequency divider and an output ofthe frequency divider; a loop filter for smoothing an output of thephase comparator and providing the smoothed signal to the voltagecontrolled osciliator; and a D/A converter for converting the output ofthe sigma-delta modulator to an analog signal and outputting the analogsignal as a control signal, wherein the voltage controlled oscillator isoperable to control the oscillation frequency based on an output of theloop filter and the control signal outputted from the D/A converter. 2.The frequency modulator according to claim 1, wherein the frequencydivider is a variable frequency divider.
 3. The frequency modulatoraccording to claim 1, wherein the voltage controlled oscillatorincludes: a frequency control terminal connected to the loop filter; anda frequency modulation terminal for inputting the control signal, andthe voltage controlled oscillator controls the oscillation frequencybased on an output of the loop filter and the control signal inputtedfrom the frequency modulation terminal.
 4. The frequency modulatoraccording to claim 3, wherein the voltage controlled oscillator whosechange of output frequency in response to a change in input frequency atthe frequency modulation terminal is smaller compared to its change ofoutput frequency in response to a change in input frequency at thefrequency control terminal.
 5. The frequency modulator according toclaim 1, wherein the control signal is connected to a junction pointbetween the loop filter and the voltage controlled oscillator.
 6. Thefrequency modulator according to claim 1, wherein a frequency bandwidthof the modulation data is wider than a bandwidth of the loop filter, anda highest frequency of the modulation data is smaller than a frequencyof the reference signal.
 7. The frequency modulator according to claim1, further comprising, on a route from the sigma-delta modulator to thevoltage controlled oscillator, an amplitude adjustment circuit foradjusting signal amplitude.
 8. The frequency modulator according toclaim 1, further comprising, on a route from the sigma-delta modulatorto the voltage controlled oscillator, a low-pass filter for removing anoise component caused in the sigma-delta modulator.
 9. The frequencymodulator according to claim 1, wherein the sigma-delta modulatoroperates by using the reference signal as a clock, and is of at least asecond order or higher.
 10. A frequency modulator that performsfrequency modulation based on digital modulation data, comprising: asigma-delta modulator for sigma-delta modulating data obtained by addinga fractional part of provided reference frequency division factor datawith modulation data; a reference frequency divider for dividing afrequency of a provided reference signal based on data obtained byadding an output of the sigma-delta modulator with an integral part ofthe reference frequency division factor data; a voltage controlledoscillator operable to control an oscillation frequency; a frequencydivider for dividing a frequency of an output of the voltage controlledoscillator; a phase comparator for comparing phases of the referencesignal frequency-divided by the reference frequency divider and anoutput of the frequency divider; a loop filter for smoothing an outputof the phase comparator and providing the smoothed signal to the voltagecontrolled oscillator; and a D/A converter for converting data obtainedby adding the output of the sigma-delta modulator with the integral partof the reference frequency division factor data to an analog signal andoutputting the analog signal as a control signal, wherein the voltagecontrolled oscillator is operable to control the oscillation frequencybased on an output of the loop filter and the control signal.
 11. Afrequency modulating method for performing frequency modulation based ondigital modulation data by using a phase locked loop having a voltagecontrolled oscillator, a frequency divider, a phase comparator, and aloop filter, the method comprising: sigma-delta modulating data obtainedby adding a fractional part of provided reference frequency divisionfactor data with modulation data; dividing a frequency of a providedreference signal based on data obtained by adding the sigma-deltamodulated signal with an integral part of the reference frequencydivision factor data and providing the frequency-divided signal to thephase locked loop; converting the sigma-delta modulated signal to ananalog signal and outputting the analog signal as a control signal; andcontrolling oscillation frequency based on an output of the loop filterand the control signal.
 12. A wireless circuit that transmits andreceives a signal frequency-modulated based on digital modulation data,comprising: a reference oscillator for generating a reference signal; afrequency modulator that operates based on the reference signal; anantenna for transmitting and receiving an electric wave; a transmissionamplifier for amplifying an output of the frequency modulator andoutputting the amplified signal to the antenna; and a receiving circuitfor processing a signal received by the antenna, wherein the frequencymodulator includes: a sigma-delta modulator for sigma-delta modulatingdata obtained by adding a fractional part of provided referencefrequency division factor data with modulation data; a referencefrequency divider for dividing a frequency of the reference signal basedon data obtained by adding an output of the sigma-delta modulator withan integral part of the reference frequency division factor data; avoltage controlled oscillator operable to control an oscillationfrequency; a frequency divider for dividing a frequency of an output ofthe voltage controlled oscillator; a phase comparator for comparingphases of the reference signal frequency-divided by the referencefrequency divider and an output of the frequency divider; a loop filterfor smoothing an output of the phase comparator and providing thesmoothed signal to the voltage controlled oscillator; and a D/Aconverter for converting the output of the sigma-delta modulator to ananalog signal and outputting the analog signal as a control signal,wherein the voltage controlled oscillator is operable to control theoscillation frequency based on an output of the loop filter and thecontrol signal, wherein, when data is transmitted, the frequencymodulator is controlled so as to perform frequency modulation based ondata to be transmitted, and wherein, when data is received, thefrequency modulator is controlled so as to output a non-modulatedsignal, and the receiving circuit processes the signal received by theantenna by using the non-modulated signal outputted from the frequencymodulator as a local signal.